Conventional control valves in process plants do not have smart positioners or other self-provided feedback mechanisms for allowing a user to tell how well such valves are performing. However, this also means that the cost of conventional control valves is low compared to smart valves. Thus, usage of conventional valves can be expected to account for the majority of valves used in process plants.
It is known that a conventional control valve may be controlled using a single loop (auto mode) control system as shown in FIG. 1 or a multiple loop (cascade mode) control system as shown in FIG. 2. The auto mode control system 1000, as shown in FIG. 1, typically comprises only one control loop 1001 which controls a conventional control valve 20 that regulates fluid flow to a number of processes 80, 90. The valve 20 is controlled by a controller 30 that sends a controller output (OPa) to the valve 20. A measurable process output (commonly referred to as a process variable (PVa)) is obtained after all the processes 80, 90 using a measurement sensor/transmitter 40 provided in the control loop 1001. A desired process output (commonly referred to as a set-point variable (SVa), a constant) is fed into a summing junction 50 provided in the control loop 1001. PVa is also fed into the summing junction 50 such that the difference of SVa−PVa is input to the controller 30 from the summing junction 50. The controller output OPa which is input to the valve 20 is thus a function of both SVa and PVa. Ultimately, the purpose of the control loop 10 is to have the PVa reach a steady state that is as close to the SVa as possible.
The cascade mode control system 2000, as shown in FIG. 2, typically comprises a number of control loops 2001, 2002 which together control a conventional control valve 20 that similarly regulates fluid flow to a number of processes 80, 90 in the process plant 2. This valve 20 is typically controlled using a primary controller 31 as well as a secondary controller 32. Where more control loops are used, the number of controllers correspondingly increases.
In the first control loop 2001, a first measurable process output or process variable (PV1) is obtained after all the processes 80, 90 using a first measurement sensor/transmitter 41 provided in the first control loop 2001. PV1 is fed into a summing junction 51. A final desired process output or set-point variable (SV1, a constant) is also fed into the first summing junction 51 such that the difference of SV1−PV1 is input to the primary controller 31 from the first summing junction 51. Output (OP1) of the primary controller 31 is thus affected by both SV1 and PV1.
In the second control loop 2002, a second measurable process output or process variable (PV2) is obtained after only one process 80 using a second measurement sensor/transmitter 42 provided in the second control loop 2002. PV2 is fed into a second summing junction 52. OP1 from the primary controller 31 is also fed into the second summing junction 52 such that the difference of OP1−PV2 is input to the secondary controller 32 from the second summing junction 52. From the secondary controller 31, a controller output (OP2) is sent directly to the valve 20. OP1 thus serves as a set-point variable (SV2) for the secondary controller 32 such that OP2 of the secondary controller 32 is affected by both OP1 and PV2. The secondary loop 2002 in a cascade mode control system 2000 thus serves to fine tune the controller input OP2 that is received by the valve 20. Ultimately, the purpose of the cascade control system 2000 is to have PV1 reach a steady state that is as close to SV1 as possible.
Understandably, unchecked and undetected deterioration in a conventional control valve can have significant impact on process control, and in extreme cases, may lead to unscheduled plant shutdown. As usage of conventional control valves in process plants worldwide will continue to be high, a reliable online/offline diagnostics tool using live/stored data for such conventional control valves will be valuable in enabling early detection of valve related problems while being a non-intrusive method that does not require plant shutdown.
At present, currently available valve diagnostics systems are only applicable to auto mode control systems as they rely on the assumption that the set-point variable that is input to the controller through the summing junction is a constant. Thus, cascade mode control loops have to be artificially set as auto mode control loops in order to use currently available valve diagnostics systems. However, this gives rise to questionable accuracy of diagnostics results for cascade mode control systems because the secondary controller of a cascade mode control system does not receive a constant set-point variable input but instead receives a variable input of OP1 from the primary controller 31.
In particular, stiction is a problem that currently available valve diagnostics systems has not been able to determine adequately for conventional control valves that are controlled in a cascade mode control system due to the variable input set-point variable received by the secondary controller. Stiction is a valve problem where the valve does not increase its flow rate despite increasing controller output OP to the valve to do so. Stiction results in the measurable process variable PV remaining relatively unchanged over a period of increasing controller output OP to the valve, this period being commonly known as the stiction band 301 as shown in FIG. 3. In this situation, the controller continues to increase its output OP in an attempt to raise the unchanging PV. Within the valve stiction there is another problem of valve jump where the valve eventually responds when the controller output OP reaches a certain value, resulting in a sudden jump 302 in the measurable process variable PV that far exceeds that which is normally expected with the input OP value. This jump typically overshoots the desired set-point variable SV for the process. When a jump occurs, the controller reacts with a sharp drop in its output OP which results in the PV correspondingly falling. To avoid the PV dropping too much below the desired SV, the controller again increases its OP, but stiction of the valve again results in the valve not correspondingly increasing flow rate until valve jump occurs where the OP reaches a certain value at which the valve finally responds with another jump in the PV beyond the desired SV. Valve stiction thus results in the PV remaining unchanged for periods of time between sudden jumps or spikes in PV about the desired SV, instead of maintaining a steady state PV at or close to the desired SV.